An apparatus of this type is known from DE-A-39 26 640 (FIGS. 1-3) and has a cylindrical knife holder in which two diametrically opposing cutting blades are inserted, which cutting blades protrude with their cutting edge over the peripheral surface of the knife holder. For the reception of the cutting blades, the knife holder is provided with radial grooves, which extend in the direction of the longitudinal axis of the knife holder and which are open towards the peripheral surface of the knife holder. Inserted in these grooves are holding bars, into which are screwed adjusting screws, which are supported by their front ends on the bottom of the grooves. Each holding bar has a projection, on which the associated cutting blade is supported by its end lying opposite the cutting edge. For the clamping of the cutting blades, clamping screws screwed into the knife holder are present, which engage on the holding bars on that side thereof which lies opposite the cutting blades.
For the radial adjustment of a newly inserted cutting blade, the adjusting screws, once the clamping screws are loosened, are screwed individually. Following completion of the adjusting operation, the clamping screws are tightened. The setting of the cutting blades is then checked on the basis of sample cuts. If need be, this adjusting operation has to be repeated one or more times until the correct setting is found. It is clear that this adjusting operation is very time-consuming. To enable the holding bars to receive the adjusting screws, which, for reasons of strength, must have a certain minimum diameter, they must be relatively wide. The effect of this is that the grooves, too, must be made correspondingly wide.